The present invention relates generally to noise reducing circuitry for radio frequency (RF) signal receivers, and more particularly to improved noise reducing circuitry that is particularly well adapted for use in single sideband (SSB) receivers.
High amplitude electrical impulse noise created by electrical machinery or lightning during electrical storms may be received by wideband antennas of RF signal receivers causing annoying interference. Although the impulse noise may be of relatively short duration, the internal circuitry of RF signal receivers can stretch impulses having a duration of tenths of microseconds to more than several milliseconds at the receiver's output. If such impulse noise is repetitive, such as, for example, spark ignition from vehicles and radiation from neon signs, the performance of an RF signal receiver can be degraded such that intelligible communications are impossible. Furthermore, the effects of such impulse noise are aggravated when the RF signal being received is relatively weak.
The problems caused by impulse noise have been alleviated somewhat in prior art receivers by using separate noise receiving circuitry tuned 2 to 3 mHz away from the operating frequency band of an RF signal receiver. Since impulse noise has a relatively wide bandwidth, noise is assumed to be present in the desired RF signal whenever detected by the separate noise receiving circuitry. Receivers including separate noise receiving circuitry are described in U.S. Pat. Nos. 3,623,144 and 3,725,674. However, such separate noise receiving circuitry is not only costly, but is also inadequate for wide band receivers since the characteristics of impulse noise may vary from one end of the band to the other.